Sugar is one form of carbohydrate produced in photosynthesizing cells in most higher plants and is the main form of transported carbon in most annual field crops such as corn, rice, soybeans and wheat. As such its movement and concentration across various plant membranes is critical to plant growth and development. In addition sugar is the main form of carbon that moves into developing seeds of soybeans, rice, corn and wheat. This movement and concentration is accomplished by the action of carrier proteins that act to transport sugar against a concentration gradient often by coupling sugar movement to the opposite vectoral movement of a proton. Specific sugar carrier proteins from these crop plants could be manipulated in efforts to control carbon flux and the timing and extent of sugar transport phenomena (e.g., grain fill duration) that are important factors in crop yield and quality. Accordingly, the availability of nucleic acid sequences encoding all or a portion of sugar transport proteins would facilitate studies to better understand carbon flux and sugar transport in plants, provide genetic tools for the manipulation of sugar transport, and provide a means to control carbohydrate transport and distribution in plant cells.
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